rhetorical question used by teenagers. "turn up" is the act of getting drunk and high and being reckless so "turn down" would mean sobering up. turn down for what is really saying i am fucked up and will continue to be all night no matter what. the only appropriate answer to this question would be "nothing"

Johnny: turn down for what ?!
Ben: The cops and my dad.
Johnny: You're a pussy! TURN DOWN FOR NOTHING

It is a phrase modern day teenagers utilize mainly on the weekends to express their genuine excitement for the upcoming events. Such events include binge drinking copious amounts of alcohol and smoking various kinds of hallucinogenics such as "blow" and "weed." All of which could lead to you possibly causing a chain reaction of people humping, trashing, and even thrashing of the numerous objects in the room and even crashing the ceiling of your apartment complex until the bottom floor. When using the phrase, be sure to proceed with caution and the resulting damage caused by turning down could lead to expensive lawsuits of undocumented aliens and even substance abuse.

Darshan was very excited for the weekend so he asked Jamal "turn down for what?"